The present invention relates to data storage devices having actuators, and more specifically, to integrated circuit technology and a method for providing closed loop self-contained control of head load/unload velocity in rotating media mass storage applications.
Data storage devices, and in particular, data storage devices of the type that accept a removable cartridge containing a disk-shaped storage medium, usually employ either a linear actuator mechanism or a rotary arm actuator mechanism for positioning the read/write head(s) of the disk drive over successive tracks of the disk-shaped storage medium. In most disk drives, and particularly in those that receive removable disk cartridges, the linear or rotary arm actuators are moved to a retracted, or parked position when the disk drive is not in use. In such a retracted position, the read/write heads of the disk drive are moved off and away from the surface(s) of the storage medium in order to prevent damage to the head(s) and storage medium. In order to resume use of the disk drive, the read/write heads must once again be loaded onto the surface(s) of the storage medium so that the data transfer can begin. It is important that the head loading operation be carried out in a controlled manner to prevent damage to the read/write heads.
Some magnetic storage devices support a head loading velocity control mechanism for a disk drive that measures the back EMF voltage across the actuator of the disk drive to obtain an indication of the velocity of the actuator. The measured back EMF voltage is then employed in a control scheme to control the velocity of the actuator during a head loading operation. Unfortunately, the circuitry needed to measure the back EMF voltage across the actuator increases the cost and complexity of the disk drive. Furthermore, this technique provides only a rough control of the actuator velocity, which may not be acceptable in many applications.
Comparatively, other magnetic storage devices utilize a velocity control technique for a disk drive actuator that employ thermal measurements to estimate the velocity of the actuator. Again, however, the circuitry necessary to obtain accurate thermal measurements unduly increases the cost of the disk drive, and this technique is susceptible to inaccuracies.
Further yet, some devices employ high-precision glass scales affixed to a disk drive actuator for obtaining accurate position and track counting information during track seek operations. See, e.g., Thanos et al., U.S. Pat. No. 5,084,791. Unfortunately, the cost and complexity of the high-precision glass scales and associated optical circuitry make them disadvantageous. Certain products in the xe2x80x9cBETAxe2x80x9d line of Bernoulli disk drives manufactured by Iomega Corporation, the assignee of the present invention, employ an optical sensor and a gray-scale pattern affixed to a linear drive actuator to obtain an indication of the linear position of the actuator. However, these products do not, and are not capable of, deriving or controlling the velocity of the actuator using the position information generated with the gray-scale pattern and optical sensor.
U.S. Pat. No. 5,615,064, to Blank et al. discloses a digital storage system in which a flying read/write head is loaded onto the surface of moving storage media with controlled velocity to avoid contact with the surface of the storage media. Head load velocity is detected by measuring the back EMF generated by the head arm actuator. Improved control and accuracy is obtained by breaking up the head arm actuator drive power into a series of pulses and measuring the back EMF induced into the low impedance voice coil of the head arm actuator in between pulses but only after the actuator current has been reduced to substantially zero in order to avoid interference by actuator current induced voltages.
FIG. 1 is a schematic diagram of the voice coil motor driving circuit and a block diagram of the control, as taught by Blank et al. The current is driven through the voice coil 119 by transistors pairs 111-113 to move the actuator arm in one direction, and by 115-117 to move the arm in the other direction. When transistors 111 and 113 are turned on, current flows from the positive terminal of the power supply, down through transistor 111, through the voice coil in a first direction from terminal 127 to terminal 129, and out through transistor 113 to the negative terminal of the power supply. When transistors 115 and 117 are turned on, current flows from the positive terminal of the power supply, down through transistor 115, through the voice coil in a second direction from terminal 129 to terminal 127, and out through transistor 117 to the negative terminal of the power supply. In this way, current can be made to flow in either direction through the voice coil, and move the actuator arm in either direction. When all four transistors 111-117 are driven such that the current in the coil decays to zero, no voltage drops occur across the coil due to resistance.
The only significant voltage across the coil 119 is due to the back EMF generated by motion of the coil through the field magnet of the motor, although there may be some voltage due to leakage currents from the drive amplifiers. This back EMF is proportional to the velocity of the motion of the arm. The EMF is amplified by amplifier 121, and fed to the control processor 123. Control processor 123 includes an analog to digital (A/D) converter for converting the analog amplified EMF from amplifier 121 to digital signals for processing according to programmed instructions in a program memory. When low EMF signals are present, the processor 123 determines that the arm 27 is moving at a low velocity, and processor 123 signals the drive circuits 125 to which processor 123 is connected to once again drive current through the coil 119 in the direction to increase the velocity of arm 27. After a calculated on-time of this drive current, the processor 123 again signals the drive circuits 125 to turn off the current to coil 119 so that a clear EMF signal can thereafter be measured, to thereby determine the velocity of arm 27 after the calculated on-time of the above-mentioned drive current. The circuit of FIG. 1 continues to operate until a signal is received in the read head from the disk that indicates that the head has been loaded onto the disk. If the head is being unloaded from the disk, the current is driven through the voice coil in the opposite direction until the arm comes to rest in the detent 19 causing the EMF to go to zero.
Referring now to FIGS. 2A and 2B, further operation of the circuit of Blank et al. is described. The drive current waveform is shown on top in FIG. 2A, and the combined drive voltage and back EMF waveform is shown below in FIG. 2B. These waveforms are generated by the control processor 123 using a simple threshold algorithm. In this regard, Blank et al. teaches to measure the actuator arm velocity using back EMF induced in the low impedance voice coil at times when drive current is not being applied. Because the drive voltage is several orders of magnitude larger than the back EMF, the voltage scale of the lower portion of the waveform of FIG. 2B is broken in the middle.
The first drive current pulse, which starts at time zero and continues until time two of FIG. 2A, provides torque at the voice coil to initiate movement of the actuator arm from its rest position. This drive current pulse is about five hundred microseconds. At time two, the current is turned off and the current is allowed to decay for about 200 microseconds so that back EMF can be measured without interference by the drive current voltage drop. As seen in the top part of the lower waveform, the back EMF has not yet reached the threshold 211 and, in fact, goes to zero just as EMF is measured at sample 217. As soon as the EMF is measured and found to be below threshold 211, the next current pulse starts at time three. Zero EMF indicates that velocity also went to zero, but also that the actuator arm has been moved out of the detent and is on a flat section of a ramp. Accelerating torque is applied by the voice coil between times three and five resulting in a velocity and proportional back EMF that nearly reaches the threshold 211, and does not decay all the way to zero by time six when the EMF is again measured and used by the control processor to apply the third drive current pulse.
At the end of the third drive current pulse, the back EMF and the proportional velocity of the actuator arm has exceeded threshold 211, and they remain above the threshold for two samples but they have already decayed to a level at or below the threshold 211 by the third sample which occurs at time ten. Accordingly another wide drive current pulse is provided beginning at time ten. This fourth pulse accelerates the arm to a velocity indicated by the back EMF signal 213 to be well above the threshold 211 during zero current time 215, and the back EMF does not decay to or below the threshold until the fifth EMF sample is taken at time fifteen. This larger velocity decay time indicates that the EMF voltage was adequately above the threshold so as to allow the process to determine that such powerful wide current pulses are no longer needed to keep the arm moving at the target load velocity and the control processor hereafter applies shorter drive current pulses of about two hundred fifty microseconds as indicated at 219.
It will be noted that during these off times, eight to ten and twelve to fifteen, the back EMF decays rather fast, as indicated by the relatively steep slope of the decay of the back EMF. This phenomenon is caused by the relatively higher friction of a tang portion on the flat portion of the ramp. As the tang portion passes the transition to the sloped section of the ramp, the back EMF decay becomes more gradual due to a reduced normal component of load force and, therefore, reduced friction, and in some cases a contribution of potential energy as the tang portion goes down the sloped section of the ramp.
Continuing at time fifteen, when the back EMF has now dropped to below the threshold 211, another drive current pulse is applied by control processor 123, but now a shorter pulse width is employed as the current turns off at time sixteen. This shorter pulse provides a shorter acceleration time and a lower end velocity at time sixteen. These steps are repeated between times seventeen and nineteen. The seventh drive current pulse in FIG. 2A has raised the velocity such that the back EMF does not decay to the threshold by time twenty-one and, therefore, more samples are taken until time twenty two.
The above-described process steps of Blank et al. are repeated as the actuator arm moves the tang portion down the slope of the ramp at a velocity that is controlled by the process to be near the threshold as measured by the back EMF. As the tang portion leaves the slope and the head begins to fly out over the disk, the tang portion to ramp component of friction disappears and, therefore, back EMF decay is much more gradual. Accordingly, at time forty, the back EMF is still above the threshold 211 when signals begin to be received from the read/write head, and actuator arm drive current control is accomplished using feedback from the disk media as is known in the prior art.
The unloading of the head from the disk media to the detent position is accomplished in a similar manner using a drive current of opposite polarity. Accordingly, the waveforms shown in FIGS. 2A and 2B will both be inverted during head unload from those shown, but the sequence of pulses is similar. Signals from the read/write head may be used to determine when the head has approached the ramp. Several consecutive current pulses followed by near zero back EMF indicate to control processor 123 that the head has been unloaded.
However, as illustrated in FIG. 1, Blank et al. requires Analog to Digital (A/D) conversion of the back EMF signal. Additionally, as illustrated in the waveforms of FIGS. 2A and 2B, Blank et al. teaches the application of a variable width pulse train as the actuator travels from the detent the flat portion of the ramp, to the slope section, etc. to load the heads, and vice versa for unloading the heads. In other words, Blank et al. requires control circuitry for generating a pulse train having different pulse widths at different times. Both A/D circuitry and the additional control circuitry for variable pulse widths add to the expense of the control loop, and consequently, Blank et al. does not teach a system having amplitude modulation of a fixed period current pulse train via back EMF feedback, whereby there is no need for A/D circuitry to monitor the back EMF.
In view of the foregoing, there exists a need for a data storage drive having the ability to process back EMF originating from the actuator coil during zero commanded current to reduce the amplitude of the commanded current pulse train and thereby control the velocity during head load. There further exists a need for an improved method that uses the analog back EMF signal to modulate pulse amplitude to control pulsed current velocity during a head load/unload process. There still further exists a need for such a system wherein there is no need to monitor back EMF as a result of a self-contained analog control circuit.
The present invention provides a method and apparatus for providing closed loop control of head load velocity in rotating media mass storage applications. A novel pulsed current velocity controlled head load/unload circuit is provided that uses the back EMF analog signal to modulate pulse amplitude. The invention provides a system having amplitude modulation of a fixed period current pulse train via back EMF feedback, whereby there is no need to monitor back EMF via A/D conversion circuitry.
Additional features and advantages of the present invention will become evident from the below description of the invention.